Enhanced toluene removal using granular activated carbon and a yeast strain Candida tropicalis in bubble-column bioreactors

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Abstract

The yeast strain Candida tropicalis was used for the biodegradation of gaseous toluene. Toluene was effectively treated by a liquid culture of C. tropicalis in a bubble-column bioreactor, and the toluene removal efficiency increased with decreasing gas flow rate. However, toluene mass transfer from the gas-to-liquid phase was a major limitation for the uptake of toluene by C. tropicalis. The toluene removal efficiency was enhanced when granular activated carbon (GAC) was added as a fluidized material. The GAC fluidized bioreactor demonstrated toluene removal efficiencies ranging from 50 to 82% when the inlet toluene loading was varied between 13.1 and 26.9 g/m3/h. The yield value of C. tropicalis ranged from 0.11 to 0.21 g-biomass/g-toluene, which was substantially lower than yield values for bacteria reported in the literature. The maximum elimination capacity determined in the GAC fluidized bioreactor was 172 g/m3/h at a toluene loading of 291 g/m3/h. Transient loading experiments revealed that approximately 50% of the toluene introduced was initially adsorbed onto the GAC during an increased loading period, and then slowly desorbed and became available to the yeast culture. Hence, the fluidized GAC mediated in improving the gas-to-liquid mass transfer of toluene, resulting in a high toluene removal capacity. Consequently, the GAC bubble-column bioreactor using the culture of C. tropicalis can be successfully applied for the removal of gaseous toluene.

Introduction

Various industrial sources that emit gaseous volatile organic compounds (VOCs) are facing stringent regulations, and packed-bed biofilters have been widely applied for the removal of VOCs from waste air streams [1], [2]. However, common operational problems such as excess biomass accumulation and loss of microbial activity make these biological treatment methods less attractive [3], [4]. These problems become more pronounced when packed-bed biofilters are subjected to high VOC loading rates over an extended period of operation, since inactive microbial constituents continuously accumulate on the surface of the packing materials [5]. In addition, the VOC removal capacities of packed-bed biofilters are commonly reduced when subjected to transient loading conditions due to the slow responses of microorganisms in the biofilm phase [6].

Suspended-culture bioreactors treating gaseous VOCs in bubble streams provide reliable operation and can be applied to overcome limitations in biofiltration processes [7], [8]. For example, higher toluene elimination capacities have been achieved in a bubble-column bioreactor compared to those reported in a compost-based biofilter [8]. Suspended-culture bioreactors are more advantageous than packed-bed biofilters when operated at high pollutant loading rates, where inactive microbial constituents accumulate rapidly and nutrient availability are not easily maintained within an optimal range. Nevertheless, the treatment of VOCs in such bioreactors strongly relies on the mass transfer of pollutants and oxygen from the gas to the aqueous phase; therefore, the mass transfer limitation needs to be carefully considered. The presence of adsorptive materials in suspended-culture bioreactors can improve the volumetric mass transfer rate of VOCs; therefore, two-phase partitioning bioreactors have been explored to overcome the mass transfer limitation [9], [10]. The use of granular activated carbon (GAC) as a second solid phase may have a beneficial effect on the overall bioreactor performance, because GAC can absorb and slowly deliver target compounds to microorganisms.

Another disadvantage of suspended-culture bioreactors is that the growth activity of VOC-degrading bacteria in a culture can be limited under acidic conditions [11] and/or due to the accumulation of harmful by-products in the liquid phase [12], which consequently causes a major obstacle in achieving stable long-term operation. Yeast cultures may offer advantages in the operation of bubble-column bioreactors because of their ability to tolerate low pH and unfavorable conditions [13]. Recent research has suggested that yeast cultures can degrade a variety of chemicals at rates equal to or greater than those observed in bacterial systems [14], [15]. Candida tropicalis, a strain of yeast, has been used for the production of microbial proteins [16] and the bio-accumulation of dyes and heavy metals [17], and has also been used for the biodegradation of phenol [18] and m-cresol [19] in waste water phases. Therefore, C. tropicalis is believed to be a potential candidate for use in bubble-column bioreactors for enhanced VOC biodegradation, even under unfavorable and transient loading conditions.

In this study, the biodegradation of toluene using a C. tropicalis culture was investigated in bubble-column bioreactors operated both in the absence and the presence of GAC. The toluene-degrading performance was monitored by determining changes in the removal efficiency when the bioreactors were operated under variable loading conditions. The mass balance was evaluated by determining toluene removal efficiencies, elimination capacities, CO2 production, and microbial concentrations of C. tropicalis. Furthermore, the effect of GAC, as the fluidized material, on the mass transfer between the gas and liquid phases was also investigated.

Section snippets

Culture of C. tropicalis

A strain of C. tropicalis was obtained from the Korean Culture Center of Microorganisms (KCCM 50075). The strain was first incubated for 3 weeks on yeast/mold (YM) agar plates before transferring to fresh media. After several cycles of regeneration, the strain was transferred into a 120-mL serum bottle containing 50 mL of a sterilized nutrient solution. The nutrient solution consisted of a hydrocarbon minimal medium (HCMM), containing 1.36 g/L KH2PO4, 1.42 g/L Na2HPO4, 0.5 g/L (NH4)2SO4, 3.03 g/L KNO

Mass transfer in bubble-column reactors

In order to evaluate the gas–liquid mass transfer characteristics, the volumetric mass transfer coefficient of oxygen (KLaoxygen) was abiotically measured in the bubble-column, either in the absence or presence of GAC. Initially, the dissolved oxygen (DO) in each bioreactor was removed by nitrogen gas sparging, and the change in DO was monitored as the air supply was resumed at an empty bed gas residence time (EBGRT) of 1 min. KLaoxygen was calculated using a dynamic mass transfer equation, as

Conclusions

The bubble-column bioreactors were operated using a strain of Candida tropicals for the treatment of a toluene-contaminated air stream. The experimental results demonstrated enhanced toluene biodegradation using GAC as a fluidized material. The enhanced toluene removal efficiency can be accounted for by the positive effect of GAC on the gas-to-liquid mass transfer. As a result, more toluene was available for use as a carbon source by C. tropicalis in the GAC bioreactor. The results from the

Acknowledgments

This study was supported by Korean Ministry of Environment as “The Eco-technopia 21 Project”.

References (26)

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